SPANX Higher Power High-Waisted Mid-Thigh Shaper 032 Amazon
This is much more comfortable and free-feeling than a traditional body-shaper. It smooths out the bulges (I'm a size 14-16, and there are plenty), and has never rolled down. For those who said it rolled down on them, the only thing I can think is that maybe they were wearing the wrong size, or perhaps have long torsos.
As for the gusset, I've never used it, and always wear underwear. Obviously, it takes a bit more effort to pull it down, but it's no big deal; I doubt it adds more than 15 seconds to my bathroom time.
All in all, an excellent product that has allowed me to comfortably wear figure-hugging clothes that I couldn't wear before.